This is a great way to add some crunch to D&D 5e. It is backward-compatible in several ways:

• Original D&D 5e characters can play alongside A5e Characters
• All of your existing D&D 5e materials other than the core books are still useful (this is the big one!)
• Alternatively, you can pick and choose which systems you wish to pull into a standard D&D 5e game

One caveat to the third item that is specific to this book: if you are going to run A5e characters, you should run an A5e game, not pick & choose which parts you like. I could see back-porting one class or archetype if you like the feel, but the A5e system has a few core new mechanics like proficiency dice that are worked in through the entire game.

I especially like the updated ranger, barbarian (berserker), and monk (adept) classes in particular as they expand each class into a wider range of archetypes.